This invention relates to a line output transformer comprising a ferromagnetic core with at least one leg, at least one high voltage winding being arranged on a leg of the core and being magnetically coupled to the core, rectifier means and capacitor means being associated with the high voltage winding for generating a smoothed DC voltage, a high voltage conductor being electrically connected to the capacitor means for applying the said DC-voltage to an electrode of a cathode ray tube.
An example of a line output transformer is known from U.S. Pat. No. 4,315,306. The combination of the high voltage winding and the rectifier means (usually a number of semiconductor diodes) generates a high DC voltage (termed EHT) that is smoothed by the capacitor means and applied to the anode of a cathode ray tube. This cathode ray tube may be the picture tube of a television receiver or a monitor. For this type of equipment there are safety regulations that specify that certain electrical fields in the vicinity of the device may not exceed certain limits because such fields could be hazardous to the health of people using these devices. For example, the AEF2, which is the term used for the alternating electrical field in band 2 (the frequency band comprising the horizontal deflection frequencies) may not exceed 2.5 V/m at a distance of 0.5 m from the centre of the screen of the tube.
In general, all pans having an alternating voltage with a frequency in band 2 will contribute to the AEF2. The back and sides of a monitor can easily be shielded by means of a metal housing or an electrically conductive coating. It is more difficult to shield the front side of the picture tube because this side comprises a glass screen. The inner side of the cone and the inner side of the screen of the picture tube are electrically conductive and are connected to the anode contact to which the high voltage conductor of the line output transformer is to be connected. Ripple on the EHT is thus connected to the inside of the screen and therefore the screen acts as a generator for the AEF2 at the front side of the picture tube. It has been found that a ripple of 15-25 V on the EHT of about 20 kV will occur in current commercially available monitors. Depending on screen size, this ripple will cause an AEF2 value that exceeds the safety requirements. For example, on a 17 inch monochrome monitor a ripple of 21 V and an AEF2 value of 4 V/m has been found. The ripple is partly due to the internal stabilization of the EHT, but the major part is caused by capacitive coupling of the flyback pulse on the horizontal deflection coils to the inner coating of the picture tube. In principle it would be possible to reduce the AEF2 value by shielding the tube screen by means of an electrically conductive coating. Although such coatings are available, they are very expensive and not suitable for mass production. Therefore, it would be preferable to eliminate or reduce the cause of the AEF2. This could be achieved by providing an electrically conductive layer between the deflection unit and the tube. However, it is difficult to obtain a good electrical isolation between this layer and the horizontal deflection coils. Furthermore, this conductive layer must have a conductivity that is sufficient to perform its shielding function, but low enough to prevent excessive eddy currents. Because it is very difficult to meet all these requirements, this solution has not been used in practice.